Electrolyte



Aug. 30, 1955 c. c. HOUTZ ET AL STABILIZATION OF ELECTROLYTIC CAPACITORS Filed June 20, 1952 FIG.

ELECTROLVTE CON TA/N/NG NI TROS O-ORGA N/C ELECTROLYTE CONFA lN/NG N/TROSO-ORGA N/C COMPOUND PORO us, 5 //v r5950 FILM FORMING METAL c. c. HOUTZ WVENTORS 0. A. McLEA/V ATTORNEY United States Patent ()fiice 2,7 l ,72 l Patented Aug. 30, 1955 2,716,721 STABILIZATION OF ELECTROLYTIC CAPACITORS Charles C. Houtz, Bernardsville, and David A. McLean,

Chatlram, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 2t), 1952, Serial No. 294,65i 15 Claims. (Cl. 317-230) This invention relates to electrolytic capacitors and more particularly it relates to electrolytic capacitors containing electrolytes embodying substances which act to These electrolytic capacitors are made up of a pair of electrodes at least one of which is made of a metal which has a dielectric oxide film, which may be formed anodically, on its surface, the oxide film being in contact with a conductive electrolyte. The deterioration of capacity and power factor of these capacitors appears to be associated with a progressive change in this oxide film during use. Thus, with tantalum electrodes, the original film is a smooth layer exhibiting an interference color depending upon the film thickness which is in turn dependent upon the voltage at which the film was formed. During 4 the use of the capacitor this oxide film gradually becomes a dull gray and the degree of deterioration of electrical properties progressively increases as the graying progresses. Analogous effects occur with electrodes made of other film-forming metals.

The present invention is based on the discovery that the stable nitroso-organic compounds, particularly those soluble to a substantial extent in the electrolytes of the capacitors, act to retard both the deterioration of electrical properties of electrolytic capacitors and the rate of graying, or analogous modification, of the oxide film referred to above. Examples of capacitors stabilized according to the present invention are shown in the accompanying drawing in which:

Fig. l is a front elevation, in section, of a rolled foil type electrolytic capacitor, sealed within a can;

Fig. 2 is a perspective view of the condenser unit of Fig. 1 outside the can and partly unrolled to show its construction; and

Fig. 3 is a front elevation, in section, of an electrolytic capacitor having a porous anode formed of sintered metal powder.

In the form of capacitor shown in Figs. 1 and 2 the condenser uit 1 is of the rolled foil type being made up of a pair of foils 2, 3 formed of a film-forming metal such as tantalum, aluminum, magnesium or beryllium having anodically formed oxide films on their surfaces and spaced from one another by double layers 4, 5 of a low density condenser paper, such as low density kraft paper. The assembly of foil and paper is rolled into a compact cylinder. Metal terminals 6, 7 preferably formed of the same metal as the foils are fastened to the ends of the respective foils as by spot welding.

The condenser unit 1, impregnated with an electrolyte containing one of the stabilizers of the present invention, is enclosed in a tubular can 8 formed of a suitable It is also often a characteristic of material such as silver or silver-plated copper. The terminals 6, 7 extend from the respective ends of the tubular can 8 passing successively through disc-shaped insulating spacers 9, 10 and cylindrical rubbery sealing plugs 11, 12. The two ends of the tubular can 8 are crimped over the respective sealing plugs 11, 12 so as to form a tight seal. The free space within the can is filled with electrolyte 13 containing one of the stabilizers of the present invention.

In the form of condenser shown in Fig. 3, an anode 14 is formed of a body of a powder of a film-forming metal, preferably tantalum, sintered about a central pin 15 which is formed of the same metal as the sintered body and which is welded to a base plate 16 also formed of the same metal. A can 17 formed of a suitable metal, such as silver, encloses said anode 14 and itself acts as the cathode. The can, which contains an electrolyte 18 containing a stabilizer of the present invention, is sealed by crimping its ends over a gasket 19 formed of a rubbery insulating material which surrounds the base plate 16 and the adjacent insulating washer 20 which may suitably be formed of a plastic. A cathode terminal lead 21 is fastened to the can 17 in any suitable manner as by soldering. An anode lead 22 is welded to the base plate 16.

The stabilizers of the present invention are effective with any of the common electrolytes which are used in electrolytic capacitors. in the rolled foil type condenser, such as shown in Figs. 1 and 2, it is common to use a viscous type electrolyte, such as an aqueous or nonaqueous glyco-borate. Thus, a suitable electrolyte can be made up of about 50 per cent ethylene glycol, 15 per cent ammonium borate and per cent water.

in a porous electrode type of electrolytic capaictor, as shown in Fig. 3, it is usually desirable to use a relatively fluid electrolyte to permit complete permeation of the pores. A suitable electrolyte for such a purpose is an aqueous solution of lithium chloride. As stated above the stabilizers of the present invention are effective with all these electrolytes and with the other electrolytes which can be used in electrolytic capacitors.

As stated above, the stabilizers of the present invention are nitroso-organic compounds. It has been established that it is the nitroso group in these compounds which is responsible for the stabilizing action so that the chemical structure of the remainder of the molecule is not of significance from the standpoint of determining whether or not stabilization will take place. However, other radicals present in the nitroso-organic compound may have different effects upon the activity of the nitroso group so that the stabilizing effectiveness of some nitroso compounds will be greater than that of others.

The stabilizers may be nitroso hydrocarbons, but preferably the compounds also contain one or more other polar groups such as hydroxyl or amino groups to increase the solubility of the stabilizer in the electrolyte. The presence of sulfonic groups is, however, to be avoided since their presence results in an attack upon the oxide film of the electrodes.

It is also necessary that the stabilizer be stable against such chemical reaction with the electrolyte as will be destructive of its nitroso-organic structure and against chemical decomposition at both the temperature of operation of the capacitor and at the temperature at which the capacitor is impregnated with the electrolyte. The nitroso-aromatic compounds, that is, the aromatic compounds having nitroso radicals bonded directly to their aromatic rings, are particularly suitable for the purposes of the present invention because of their high chemical stability. Those nitroso-aliphatic compounds which are stable under the =3 conditions set forth above are also suitable for the purposes of the present invention.

The following tables will illustrate the stabit' of capacity and power factor and the reduction ot direct-current leakage which is achieved by the add..ion of nitroso-organic compounds to the electrolytes of electrolytic condensers. In Table 1 below results are given for a series of tests run on electrolytic capacitors having tantalum foil electrodes separated by two layers of low density kraft paper each one-half mil in thickness. The electrodes were formed under a direct-current forming voltage of 200 volts at 100 C. in an aqueous glycoborate electrolyte. As is conventional, forming was continued until the leakage current decreased to a minimum value.

The capacitors were impregnated with the aqueous electrolyte made up of ethylene glycol, ammonium aerate and water which was described above. The capacitors were placed on test under a potential of 150 volts at 85 C. for 2,000 hours.

Measurements were made at the end of the test of the loss of capacity in per cent of the initial capacity at the start of the test. Measurements were also made of the power factor at the end of the test and of the peak value of direct-current leakage current reached during the test. The measurements of capacity and power factor were made at 60 cycles. Tests were made on capacitors con taining no added stabilizer and containing varying amounts of different stabilizers as shown in the table.

Table 1 Loss of Peak of D. 0.

Power Additive papac' Factor, g Dmity, Per- Percent mg lest,

cent Mieroamperes None 41. 8 8.0 25. 4 2% 2 nitroso 1 naphthol. 7. 4 4. 5.1 2% p-nitroso diphenylamin 11. 5 9.0 T. O

In Table 2, below, are recorded the results of tests on condensers which were similar except that the formation was carried out at 150 volts at 200 C. and except that the test was carried on for 1,000 hours.

Representative proportions of the added stabilizers are given in the tables above. It is ordinarily desirable that at least .05 per cent by weight and preferably at least 1 per cent by weight of the stabilizer be added to the electrolyte. A desirable range of proportions of stabilizer is from 2 per cent to per cent or to the limit of solubility in the electrolyte. It ordinarily serves no useful purpose to add a greater amount of stabilizer than is soluble in the electrolyte at the temperature of impregnation. gardless of solubility, the stabilizers will ordinarily not be added in amounts greater than per cent, although larger amounts may obviously be added if desired. The stabilizers are ordinarily added by dissolving them in the electrolyte before the electrolyte is added to the capacitor. In the foil type capacitors the impregnation of the capacitor with the stabilized electrolyte is ordinarily accomplished at an elevated temperature, such as about 110 C., with the intermittent application of partial vacuum. An additional amount of the stabilized electrolyte is ordinarily also added to the container in which the impregnated capacitor is placed. In the s'mtcred anode capacitor, the impregnation of the porous anode can also be accomplished in a similar manner, as by the intermittent application of a partial vacuum, using an electrolyte at a temperature of about C.

The invention has been described above in terms of specific embodiments and, since certain modifications and equivalents may be apparent to those skilled in the art, this description is intended to be illustrative of, but not necessarily to constitute a limitation upon, the scope of the invention.

What is claimed is:

1. An electrolytic capacitor comprising a pair of electrodes at least one of which is formed of a film-forming metal having a dielectric surface coating of an oxide of said metal and a glyco-borate electrolyte in contact with said electrodes, said electrolyte having dissolved therein between .05 per cent and 10 per cent by weight of para nitroso diphenylamine.

2. An electrolytic capacitor as described in claim 1 wherein the film-forming metal is tantalum.

3. An electrolytic capacitor as described in claim 1 wherein the film-forming metal is aluminum.

4. An electrolytic capacitor comprising a pair of foil electrodes formed of a film-forming metal having a dielectric surface coating of an oxide of said metal, said foils being spaced by sheets of porous insulating material impregnated with a glyco-borate electrolyte having dissolved therein between .05 per cent and 15 per cent by weight of a nitroso-organic compound formed of a hydrocarbon having substituted thereon as its sole substituents a nitroso group and at least one substituent selected from the group consisting of hydroxyl groups and amino groups.

5. An electrolytic capacitor as described in claim 4 wherein the nitroso-organic compound is formed of a hydrocarbon having substituted thereon a nitroso group and an amino group.

6. An electrolytic capacitor comprising a pair of electrodes at least one of which is formed of a film-forming metal having a dielectric surface coating of an oxide of said metal and a glyco-borate electrolyte in contact with said electrodes, said electrolyte having dissolved therein between .05 per cent and 15 per cent by weight of a nitroso-organic compound free from sulfonic groups.

7. An electrolytic capacitor as described in claim 6 wherein the nitroso-organic compound is a nitrosoaromatic compound.

8. An electrolytic capacitor as described in claim 7 wherein the electrodes are foil electrodes which are spaced by sheets of porous insulating material impregnated with the electrolyte.

9. An electrolytic capacitor as described in claim 8 wherein the electrodes are formed of tantalum.

10. An electrolytic capacitor as described in claim 8 wherein the electrodes are formed of aluminum.

11. An electrolytic capacitor as described in claim 6 wherein the nitroso-organic compound is a nitroso-phonol.

12. An electrolytic capacitor as described in claim 6 wherein the nitroso-organic compound is a nitrosonaphthol.

13. An electrolytic capacitor as described in claim 6 wherein the nitroso-orvanic compound is 2 nitroso l naphthol.

14. An electrolytic capacitor as described in claim 6 wherein the nitroso-organic compound is nitroso-phenyl hydroxylamine.

15. An electrolytic capacitor as described in claim 6 wherein the nitroso-orgnnic compound c .c utrosamine.

References Cited in the file of this patent UNITED STATES PATENTS 1,996,730 Thomas et al. Apr. 2, 1935 2,368,688 Taylor Feb. 6, 19- 5 FOREIGN PATENTS 416,049 Great Britain Sept. 3, 1934- 

1. AN ELECTROLYTIC CAPACITOR COMPRISING A PAIR OF ELECTRODES AT LEAST ONE OF WHICH IS FORMED OF A FILM-FORMING METAL HAVING A DIELECTRIC SURFACE COATING OF AN OXIDE OF SAID METAL AND A GLYCO-BORATE ELECTROLYTE IN CONTACT WITH SAID ELECTRODES, SAID ELECTROLYTE HAVING DISSOLVED THEREIN BETWEEN .05 PER CENT AND 10 PER CENT BY WEIGHT OF PARANITROSO DIPHENYLAMINE. 